1. Field of the Invention
The present invention relates to data communication, and more particularly, to a data communication apparatus and method based on orthogonal frequency division multiple access (OFDMA).
2. Description of the Related Art
In a time division multiple access (TDMA) method as a conventional data communication method, a plurality of time slots are defined within a data frame and the time slots are appropriately allocated to different users so that the users can share a common physical channel. When the conventional TDMA method is applied to a voice-centered communications environment, multiple access can be easily accomplished since a necessary communication speed is constant. However, in a multimedia communications environment requiring a high-speed data service as well as a low-speed voice service, a data transmission speed needs to be changed to satisfy a user's request. Since a data transmission speed can be adjusted only by changing time slots in the conventional TDMA method, the conventional TDMA method is not appropriate to a multimedia communications environment.
A direct sequence-code division multiple access (DS-CDMA) method as another conventional data communication method allows a plurality of users to simultaneously transmit/receive signals at the same time. The conventional DS-CDMA method uses a multiple access method in which users use orthogonal codes, in order to separate different users' signals from one another. In an environment in which serious multi-path fading does not occur in wireless channels, fading can be effectively overcome using a RAKE receiver according to the conventional DS-CDMA method. In addition, in the conventional DS-CDMA method, a data transmission speed can be easily adjusted by adjusting a spread factor (SF) of a code multiplied by a signal. However, compared to other data communication methods, the conventional DS-CDMA method has lower spectrum efficiency. Moreover, if a spreading factor is decreased in order to transmit data at a high speed, reliability of communications decreases. Accordingly, it is difficult to transmit data at a high speed in the conventional DS-CDMA method. In the conventional DS-CDMA method when a user transmits data using different codes, although a data transmission speed can be increased, a signal power needs to be reduced or nonlinear distortion occurs since a peak to average rate (PAR) of a transmitting signal increases, thereby decreasing reliability of a receiving signal.
A still another conventional data communication method is a TDMA method based on orthogonal frequency division multiplexing (OFDM) or a frequency division multiple access (FDMA) based on OFDM. In conventional data communication methods based on OFDM, inverse fast Fourier transform (IFFT) is performed on a transmitting signal in units of blocks, and a cyclic prefix is added to each block before the signal is transmitted. Since a cyclic prefix is inserted between adjacent blocks of an OFDM signal, inter-block interference occurring due to fading can be prevented. In addition, since channel distortion can be easily compensated for using a simple 1-tap equalizer after FFT, conventional data communication methods based on OFDM are suitable to high-speed communications having a lot of channel interference. However, conventional data communication methods based on OFDM have a large PAR and decreases the reliability of a receiving part due to nonlinear distortion caused by a high power amplifier (HPA) of a transmitting party, thereby limiting HPA power in order to reduce nonlinear distortion. Accordingly, conventional data communication methods based on OFDM which are applied to two-way wireless communications reduce the efficiency of mobile terminals and increase the manufacturing cost of mobile terminals. Moreover, when there is a carrier frequency offset, interference occurs in a receiving signal proportional to the size of an FFT/IFFT block in conventional data communication methods based on OFDM, thereby decreasing the performance of a communications system.